The invention relates to a method of forming a mechanical connection to an object, to an assembly for forming a mechanical connection to an object, and to an object to which a mechanical connection has been made.
It is known to make a mechanical connection to an object by attaching a collar to the external surface of the object, through which a force can be applied to the object in a direction parallel to the surface. This connection technique is used in particular to connect elongate objects which have a constant cross-section to one another, for example tubes (which term shall include pipes) and rods. For example, U.S. Pat. No. 3,484,123 discloses a tube coupling assembly comprising a collar which is rigidly fixed to a first tube by swaging the tube (involving expanding it radially), and a union member which is rigidly fixed to the second tube, also by swaging. The union member extends beyond the end of the second tube and has a threaded external surface. The assembly includes a nut which has a radially inwardly projecting flange at one end which abuts the collar, and which is threaded internally at its other end for engaging the threaded surface of the union member, so that rotation of the nut relative to the union member forces the two tubes towards one another.
More recently, it has been proposed to use a collar formed from a shape memory alloy, in order to avoid the need to swage an object to join the object to the collar. Shape memory alloys exhibit a shape memory effect as a result of their ability to transform between martensitic and austenitic phases. The transformation may be caused by a change in temperature: for example, a shape memory alloy in the martensitic phase will begin to transform to the austenitic phase when its temperature increases to a temperature greater than A.sub.s, and the transformation will be complete when the temperature is greater than A.sub.f. The reverse transformation will begin when the temperature of the alloy is decreased to a temperature less than Ms and will be complete when the temperature is less than M.sub.f. The temperatures M.sub.s, M.sub.f, A.sub.s and A.sub.f define the thermal transformation hysteresis loop of a shape memory alloy. An article may be formed in a desired configuration while in its austenitic phase. If it is then cooled so that it transforms to the martensitic phase, it can then be deformed by up to about 8%. The strain imparted to the article is recovered when the article is subsequently heated so that it transforms back to the austenitic phase. Further information is available in the article by L. M. Schetky in Scientific American, Volume 241, pages 68 to 76 (1979) entitled Shape Memory Alloys.
The use of a shape memory alloy collar has the advantage that significantly less time and less skill are required on the part of the installer than are required to affix a collar by swaging. Furthermore, the outlay in equipment required to install a shape memory alloy collar is significantly less than that required to install a collar by welding.
It is common for the collar to have at least one groove formed in its internal surface, the or each groove being defined longitudinally by two radially extending circumferential formations. Swaging the object, or heating a shape memory alloy collar to cause it to shrink, to cause the object and the collar to engage one another involves forcing the formations into the external surface of the object, so that a circumferential portion of the object is forced into the groove in the internal surface of the collar. This increases the pressure which the seal between the collar and the object is able to withstand.
The formations may be the end walls of the groove which is formed in the otherwise uniform internal surface of the collar, as in the collar described in US-3484123. Alternatively, the formations may be in the form of discrete circumferentially extending teeth which stand proud of the internal surface of the collar, as have been used in shape memory alloy collars.
The force which is exerted radially inwardly by a shape memory alloy collar when it shrinks is dependent on, amongst other things, the wall thickness of the collar. It is, however, desirable to minimize the wall thickness of the collar so that the assembly used to make the connection to the object occupies as little space as possible.